Conventionally, in such a reversible engine, a primary current is short-circuited at the highest voltage thereof to induce a high voltage secondary current in an ignition coil, and the secondary current is discharged from an ignition plug. This ignition timing is selected to be a position that is before a piston reaches a top dead center, which corresponds to a rotational position of the crankshaft at a predetermined angle before the top dead center of the piston. In this case, a voltage curve shows, as indicated with a continuous line in FIG. 10 for example, the highest voltage at a position of angle .theta.1 before a top dead center 0, and the negative lowest voltage at a position of angle .theta.2 after the top dead center. In this case, an angle of ".theta.1+.theta.2" is determined by the structure of a magnet and an electromotive coil. The angle .theta.1 shall be set to a predetermined value in accordance with a predetermined ignition timing due to the performance of the engine. On the other hand, the angle .theta.2 has no influence on the performance, so that there is no need to particularly set the angle .theta.2 and, therefore, no consideration is made on the angle .theta.2. If this engine is reversely rotated, its voltage curve shows, as indicated with a dotted line in FIG. 10, the highest voltage at a position of angle .theta.2 before the top dead center and the negative lowest voltage at a position of angle 81 after the top dead center. Therefore, if the engine is simply rotated reversely, a timing of the highest voltage will not be proper so that it may not be ignited at a correct timing. Namely, the engine cannot be reversely rotated under this state.
Generally, depending on the kind of work, a working shaft shall be rotated normally or reversely. For example, in installing posts of transmission lines of electric facilities, many bolts and nuts are used. To fasten the nuts, a nut runner is used. If the nut runner is of electric type, a cord for a motor may hinder the work so that it may be advantageous to use an engine as a driving source. In fastening and unfastening the nuts, rotation of the nut runner shall be switched to normal or reverse rotation. In this case, .it is generally difficult, as described in the above, to change the rotational direction by the engine itself. Therefore, a separate normal-reverse switching unit is needed. However, the unit may complicate the structure, increase the cost, and make heavier the weight to prevent the nut runner, a portable working machine, from being used at a high place.